Allow inputting wildcards
Allow an optional parameter to get_candidates

This will be useful to implement Quick without duplicating the code.
First attempt at a Quick implementation

For now, it's basically the same UX as Cangjie, except that candidates
are automatically fetched every 2 input characters, and that the space
key serves as page-down.

We'll specialize the two UX later on, based on what users expect, or
what we can do to make it even better.